Tubes and pipes are widely used products that can be manufactured by various processes. They can be made with a seam by bending a flat sheet and welding it or seamless. Among the seamless processes, rotary piercing, in which a mandrel is forced into a hot round billet set in rotation by two rolls, is used to manufacture steel tubing for critical applications such as oil drilling. This process is also used to make tubes (mechanical tubing) which are later cut and machined into a variety of mechanical parts, such as the races of ball bearings. After initial piercing, the tube is rolled down to its final diameter and wall thickness during operations in which the tube is usually in rotation and translation. Well-defined specifications of diameter and wall thickness are generally required, particularly in the case of mechanical tubing. It is obviously desirable to perform this dimensional control on-line, just after processing, which means at a stage where the tube is at elevated temperature and in motion. The traditional approach of removing a tube from the line to section it and to cool it rapidly for measuring its dimensions with a conventional thickness gauge, is time consuming and does not provide real-time information on all the produced tubing. For outer diameter measurement, optical gauges, using in particular a scanning laser beam, have been developed and are commercially available. For the determination of thickness, penetrating radiation, such as y-rays, and ultrasonics can be used, at least in principle. Systems based on y-rays have been developed using several y sources and detectors encircling the tube and provide by a tomographic reconstruction algorithm thickness mapping of the tube. Systems of this kind are in particular available from IMS Measuring Systems, Inc., 108 Blue Ridge Drive, Cranberry Twp., Pa. 16066, USA. The tested tube, in this case, should be in linear motion and not rotating. These systems are somehow sensitive to the exact location of the tube inside the gauge and are not easily movable. Another drawback is the use of radioactive materials. Ultrasonic determination of thickness does not have these limitations and is based on the measurement of the time-of flight between the echoes produced by the ultrasonic wave reverberating within the tube wall. Knowing the ultrasonic velocity from calibration (this velocity is a function of the material itself and of its temperature, which can be in principle measured by pyrometry), the wall thickness can be determined.
Ultrasonics uses generally piezoelectric transducers for the generation and detection of ultrasound, but these devices cannot be used in the case of a very hot (typically 1000 degrees Celsius) product. Non-contact generation and detection is required. Although electromagnetic transducers (called EMATs) have been developed for this purpose, they require close proximity to the tested part and tube guidance and are not used in practice for these reasons. A practical solution to ultrasonic coupling is provided by the generation and detection of ultrasound with lasers (laser-ultrasonics). Two lasers are used, one for generation, which gives a short and intense pulse and another one for detection, which is very stable and has a pulse sufficiently long to capture several ultrasonic echoes. Generation of ultrasound proceeds from the transient surface heating and material ablation produced by the generation laser. Detection is performed by the detection laser, which, when associated to an optical interferometer, senses the small surface motion produced by the ultrasonic wave reverberating within the material. The detection principle, which can be used in particular for tube wall thickness measurement, has been described in various U.S. patents by applicant and associates: J.-P. Monchalin, "Optical Interferometric Reception of Ultrasonic Energy", U.S. Pat. No. 4,659,224, issued Apr. 21, 1987; R. Heon and J.-P. Monchalin, "Optical detection of a surface motion of an object using a stabilized interferometric cavity", U.S. Pat. No. 5,137,361, issued Aug. 11, 1992; J.-P. Monchalin, "Broadband optical detection of transient surface motion from a scattering surface", U.S. Pat. No. 4,966,459, issued Oct. 30, 1990; J.-P. Monchalin and R. K. Ing, "Broadband Optical Detection of Transient Motion from a Scattering Surface", U.S. Pat. No. 5,131,748, issued Jul. 21, 1992. The feasibility of using laser-ultrasonics for the on-line measurement of the wall thickness of tubes has been demonstrated by applicant and coworkers in a tube mill and is described in the following publications: J. -P. Monchalin, "Progress towards the application of laser-ultrasonics in industry", Review of Progress in Quantitative NDE, eds D. O. Thompson and D. E. Chimenti, vol 12A, pp. 495-506, Plenum Press, 1993; J.-P. Monchalin, A. Blouin, D. Drolet, P. Bouchard, R. Heon, C. Padioleau, "Wall thickness Measurement of Tubes an Eccentricity Determination by Laser-Ultrasonics", 39th Mechanical Working & Steel Processing Conference, Iron & Steel Society, Indianapolis, In., Oct. 19-22, 1997, Iron & Steel Society, Warrendale, Pa., Vol. XXXV pp. 927-931. In this demonstration, the tube was in linear motion and not rotating, so the measurement was performed only along a line. The generation and detection spots were also superimposed, which, as we discovered later, has the consequence of producing additional noise on the detected signal.
Industry requires more than the mere measurement along a single line at the surface of a tube. Full mapping of the thickness profile throughout the tube surface is required, this being in particular needed for the determination of tube eccentricity. An obvious solution would be to use for this purpose several laser ultrasonic systems, as described in the communication entitled "Laser Ultrasonics in Industry" presented by M. Paul, A Hoffman, G. J. Deppe and L. Oesterlein at the 7th European Conference on Nondestructive Testing, Copenhagen, 26-29 May 1998. Since the cross section of a tube is not usually delimited by two offset circles, but has a more complex profile, appropriate mapping requires a sufficient number of measurement locations around the circumference, and as an example, ten may be used. Therefore, ten systems would be needed resulting in an enormous complexity and prohibitive cost. The present invention provides a practical solution to this problem, which does not require the use of multiple laser-ultrasonic systems.